A cold, dense cloud in the Milky Way’s interstellar medium is about four to five orders of magnitude denser than its diffuse counterparts, and a team of astronomers from Boston University, Harvard University, and Johns Hopkins University has found evidence that two to three million years ago, our solar system encountered one of these dense clouds, which may have been so dense that it disrupted the solar wind.

Most stars generate winds that move through the surrounding interstellar medium.

This motion creates a cocoon that protects the planet from interstellar material. The Sun’s cocoon is the heliosphere.

It’s made up of a constant stream of charged particles called the solar wind, which extends far beyond Pluto, enveloping the planet in what astronomers call a “local bubble.”

It protects us from radiation and galactic rays that can alter DNA, and scientists think it’s part of the reason why life on Earth evolved.

A cold interstellar cloud compressed the heliosphere, temporarily placing Earth and other planets in the solar system outside of its influence, according to a new study.

“Our paper is the first to quantitatively show that there was an encounter between the Sun and something outside our solar system that affected Earth’s climate,” said Professor Merab Auffar of Boston University.

“Stars move, and this paper shows that not only do they move, but they undergo dramatic changes.”

To study this phenomenon, Professor Orpher and his colleagues essentially went back in time and used advanced computer models to visualize where the Sun was located two million years ago, along with the heliosphere and the rest of the solar system.

They also mapped the path of a “localized cold cloud ribbon” system, a series of large, dense and very cold clouds made mainly of hydrogen atoms.

Their simulations showed that one of the clouds near the edge of the ribbon, a “local cold cloud,” may have collided with the heliosphere.

If this had happened, Earth would have been fully exposed to interstellar matter, where gases and dust would have mixed with atomic elements left over from the exploded star, such as iron and plutonium.

Normally, the heliosphere filters out most of these radioactive particles, but without protection they could easily reach Earth.

This is consistent with geological evidence showing increased levels of the isotopes iron-60 and plutonium-244 in the oceans, the moon, Antarctic snow and ice cores from the same period, according to the paper.

This timing also coincides with temperature records indicating a cold period.

“It is rare for our cosmic neighbors outside our solar system to have an impact on life on Earth,” said Harvard University professor Avi Loeb.

“It’s exciting to discover that our passage through dense clouds millions of years ago may have exposed the Earth to much greater amounts of cosmic rays and atomic hydrogen.”

“Our findings open a new window into the evolution of life on Earth and its relationship with our cosmic neighbours.”

“External pressure from localized lynxes of cold clouds could have continuously blocked the heliosphere for hundreds to millions of years, depending on the size of the cloud.”

“But as soon as Earth left the cold cloud, the heliosphere engulfed all the planets, including Earth.”

“It’s impossible to know exactly what effect the cold clouds had on the Earth, such as whether they caused ice ages.”

“But there are other cool clouds in the interstellar medium that the Sun likely encountered in its first few billion years.”

“And we’ll probably encounter many more over the next million years or so.”

The authors are currently working to determine where the Sun was 7 million years ago, and beyond.

Pinpointing the position of the Sun and cold cloud systems millions of years ago is made possible by data collected by ESA’s Gaia mission, which has produced the largest 3D map of the galaxy ever, showing in unprecedented detail how fast stars move.

“This cloud is certainly from our past, and if we passed through something this massive, we would have been exposed to interstellar material,” Prof Auffar said.

“This is just the beginning. We hope this paper opens the door to further exploration of how the solar system was influenced by outside forces in the ancient past, and how these forces may have shaped life on Earth.”

of paper Published in today’s journal Natural Astronomy.

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M. Offer othersIt is possible that Earth was directly exposed to cold, dense interstellar material 2 to 3 million years ago. Nat AstronPublished online June 10, 2024; doi: 10.1038/s41550-024-02279-8